Strange problem with a Fender Blues Junior

Cassiel · 15th February 2012, 01:08 PM

This is a push pull amp with a pair of EL84s. One tube has 70 volts 'missing' from the plate. I have changed the tube and it makes no difference. Bias voltage and screen voltage are normal but.....where do those 70 volts get lost?

320 volts depart B+ on their way to the plates. One plate gets 317V, the other one only gets 250V. Schematic says there's only two parts between B+ and the tube's plate. The OT and a diode to ground. The diode seems fine, the OT measures 190 Ohms across the primary winding, 95 Ohms between B+ and each plate. Looks OK too. I have no idea what's going on here, with no tubes both plate pins show the same readings. The tube is not the problem.

http://www.blueguitar.org/new/schem/fender/blues_jr.gif

Osvaldo de Banfield · 15th February 2012, 01:17 PM

Could you see if the amplifier is oscillating? In this case, a VOM can give a false reading.

Good luck.

Cassiel · 15th February 2012, 01:32 PM

Thanks. I don't have the gear to check that out but when I attach the VOM probes to the circuit this strange noise comes up, it could be oscillating....but why everything reads normal except at the plate of that tube? Could be the OT?
It would be odd but maybe with current through the windings the OT starts misbehaving. A wild guess, I know.

firechief · 15th February 2012, 01:35 PM

With no signal applied, measure the voltage on pin 2 of each EL84 they should both be aprox -10 V dc. If not check the values of R20 through R23. One tube is drawing more current than the other. You could also reverse the position of the two outputs and see if the low voltage follows the tube. Frankly it sounds like a bad tube. Good luck.

Osvaldo de Banfield · 15th February 2012, 01:39 PM

Try removing the previous tube stage and re-read the voltages. This way, the amplifier must be stop oscillating. The reason you see only one plate at abnormal voltage depend of the waveform of the oscillation and its amplitude.

Cassiel · 15th February 2012, 01:51 PM

Quote:

Try removing the previous tube stage and re-read the voltages.

That did it.

Thank you very much, everything reads OK now.

Osvaldo de Banfield · 15th February 2012, 02:09 PM

OK, well go back into the circuit to find the fault.

Cassiel · 15th February 2012, 02:29 PM

I'm not going back. Amp plays fine and it's not mine. It only oscillates when my hand gets near the circuit. Not very stable.

A friend plays a gig next week and he wants no surprises so he ask me to take a look and see if everything was OK with the amp. I did take a look and took some measurements and that's it. He's not getting any more work from me.

Again, thanks for the help.

Osvaldo de Banfield · 15th February 2012, 02:51 PM

OK.

15th February 2012, 01:08 PM	#1
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	Strange problem with a Fender Blues Junior This is a push pull amp with a pair of EL84s. One tube has 70 volts 'missing' from the plate. I have changed the tube and it makes no difference. Bias voltage and screen voltage are normal but.....where do those 70 volts get lost? 320 volts depart B+ on their way to the plates. One plate gets 317V, the other one only gets 250V. Schematic says there's only two parts between B+ and the tube's plate. The OT and a diode to ground. The diode seems fine, the OT measures 190 Ohms across the primary winding, 95 Ohms between B+ and each plate. Looks OK too. I have no idea what's going on here, with no tubes both plate pins show the same readings. The tube is not the problem. http://www.blueguitar.org/new/schem/fender/blues_jr.gif __________________ Good sound comes and goes.

15th February 2012, 01:17 PM	#2
Osvaldo de Banfield diyAudio Member Join Date: Dec 2011 Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina	Could you see if the amplifier is oscillating? In this case, a VOM can give a false reading. Good luck. __________________ A veces pongo un circo y me crecen los enanos... "Que se llama soledad", Joaquin Sabina, el capo.

15th February 2012, 01:32 PM	#3
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	Thanks. I don't have the gear to check that out but when I attach the VOM probes to the circuit this strange noise comes up, it could be oscillating....but why everything reads normal except at the plate of that tube? Could be the OT? It would be odd but maybe with current through the windings the OT starts misbehaving. A wild guess, I know. __________________ Good sound comes and goes.

15th February 2012, 01:35 PM	#4
firechief diyAudio Member Join Date: Sep 2008 Location: Near Seattle Wa	With no signal applied, measure the voltage on pin 2 of each EL84 they should both be aprox -10 V dc. If not check the values of R20 through R23. One tube is drawing more current than the other. You could also reverse the position of the two outputs and see if the low voltage follows the tube. Frankly it sounds like a bad tube. Good luck. __________________ "You can keep your insurance baby nothing is guaranteed" -Tom Petty

15th February 2012, 01:39 PM	#5
Osvaldo de Banfield diyAudio Member Join Date: Dec 2011 Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina	Try removing the previous tube stage and re-read the voltages. This way, the amplifier must be stop oscillating. The reason you see only one plate at abnormal voltage depend of the waveform of the oscillation and its amplitude. __________________ A veces pongo un circo y me crecen los enanos... "Que se llama soledad", Joaquin Sabina, el capo.


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

15th February 2012, 02:09 PM	#7
Osvaldo de Banfield diyAudio Member Join Date: Dec 2011 Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina	OK, well go back into the circuit to find the fault. __________________ A veces pongo un circo y me crecen los enanos... "Que se llama soledad", Joaquin Sabina, el capo.

15th February 2012, 02:29 PM	#8
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	I'm not going back. Amp plays fine and it's not mine. It only oscillates when my hand gets near the circuit. Not very stable. A friend plays a gig next week and he wants no surprises so he ask me to take a look and see if everything was OK with the amp. I did take a look and took some measurements and that's it. He's not getting any more work from me. Again, thanks for the help. __________________ Good sound comes and goes.

15th February 2012, 02:51 PM	#9
Osvaldo de Banfield diyAudio Member Join Date: Dec 2011 Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina	OK. __________________ A veces pongo un circo y me crecen los enanos... "Que se llama soledad", Joaquin Sabina, el capo.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
New Problem with Fender Frontman 212R	diynow89	Instruments and Amps	16	22nd November 2011 01:27 AM
Preamp issue on Fender Blues Jr.	Rotus623	Tubes / Valves	15	24th January 2011 03:13 PM
Sunfire True Sub Junior Problem	sjdemark	Subwoofers	2	15th July 2009 03:31 PM
Fender Blues Deluxe cab built HELP!	chucknorris	Swap Meet	0	7th September 2008 12:18 AM
blues junior complaints and 6G15 question	layt	Tubes / Valves	4	1st July 2006 11:38 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read